We propose to study the role of calcium in visual excitation. Preliminary observations, already made by the principal investigator, are described in this application and have been submitted for publication. The research proposed here will involve investigation into the underlying molecular interactions between retinal rod disk membranes and calcium, ATP, and sulfhydryl reducing agents. We will improve our assay for calcium binding by determing the best method of preparation of the membranes, and the optimum conditions for the interactions to occur and be observed. We will study the role and effects of nucleotides, divalent cations, reducing agents, cyclic-AMP, illumination, membrane modifications, and many other parameters on the relationship between calcium and these highly specialized membranes. It is our hope that an investigation of this type will provide key information on the missing link between photon capture by rhodopsin and the sodium conductance change leading to alterations in polarity and the consequent effects on the receptor potential.